So having been pretty committed to NFC (I have a mifare ultralight tag from dangerousthings implanted in my left hand) I have had to do a fair amount of research.
I have tested my implant android phones, windows mobile phones, blackberry. I have access to 50+ phones at any given time and have tested multiple chip types on many of them. Sadly my implant only works on NXP based phone reader which many new phones don't have.
Compatibility basically comes down to which chipset the phone uses to read chips. Most phones have either a broadcom or NXP based NFC chipset. Most of small form factor NFC tags in the wild (including the one in my hand) are Mifare ultralights which sadly do not conform to the NFC Forum spec. As a result of this only readers with NXP chipsets can read them.
Many new and popular phones like the Nexus 4 and Nokia Lumia have chipsets which only adhere to the NFC Forum spec and are unable to read data from the NXP Mifare chips. (I can only assume there are legal reasons to not support Mifare but this seems a topic of much speculation)
Currently the only small form factor NFC chip in the wild that I have tested that appears to work on every device (NFC, NXP, various Arduino NFC shields, usb readers, etc) is the NTAG203 which is NFC Forum 2 compliant, though much harder to find currently.
Amal of dangerousthings.com ran into these headaches as well and is in the process of switching over to an NTAG203 to address compatibility issues with the current line of implants. I tested a friends NTAG203 prototype and can confirm it seems to be readable by every device he has thrown at it and Amal has somehow packed the antenna small enough to be the same tic-tac size as the old mifare ultralight based implants.
So the big question is: Will the NFC ring be Mifare or NTAG203... or is it something from scratch and if so is it Forum compliant?
Perhaps this was discussed elsewhere and I just missed it.